It is well known to detect the level of current which flows from a supply voltage via feed resistors to a two-wire telephone line in order to determine the state of the line, i.e. whether the line is idle with the wires relatively disconnected, or active with the wires interconnected, at its end remote from the detector. Such detection arrangements suffer from a disadvantage in that the detection of the current, and hence of the state of the line, can be adversely influenced by common mode alternating currents induced on the line, for example from power lines. The induced a.c. can have large magnitudes, producing voltage swings of the order of 50 volts or more.
In order to reduce the sensitivity of such known line detection arrangements to induced a.c., it is known to couple the detector to the line via a filter. However, such a filter must be relatively transparent to d.c. signals while suppressing induced a.c. at frequencies of 50 or 60 Hz. Accordingly, the filter tends to be complicated and adds undesirably to costs and space requirements.
An object of this invention is to provide an improved two-wire telecommunications line detection arrangement.